Due to the compact size, low prices, high power, etc., semiconductor laser devices are used in diverse technical fields including IT technologies, such as communication and optical disks, as well as medicine, illumination, etc. In recent years, among others, development of gallium nitride (GaN)-based semiconductor laser devices having a wavelength of 405 nm for the Blu-ray Disc format is actively underway. Development of pure-blue laser devices using GaN-based semiconductor and having a wavelength of 450 nm-470 nm used for laser displays, backlights in liquid crystal displays, etc., is also underway.
If the shape of radiation beam has multiple peaks, a write operation to an unintended location etc. may occur in disk applications. In addition, in display and backlight applications, the optical system to shape emitted laser light becomes complex, which may result in a cost increase. As such, an output-beam shape whose far-field pattern (FFP) is single peaked is required of GaN-based semiconductor laser devices.
If the planarity of the optical waveguide is low, and there are surface undulations having a period to cause scattering of the laser light guided in the optical waveguide, a part of the laser light subjected to scattering is absorbed in the resonator. In addition, a part of the scattered laser light is emitted to the substrate side. Since scattering causes optical loss, the efficiency of the laser device is reduced. The light emitted to the substrate side is guided in a mode called “substrate mode” which is different from an expected guided mode. If this light is emitted to the outside, a ripple is caused to appear in the FFP of the output beam, and thus the single-peaked nature is degraded. Therefore, in order to achieve a semiconductor laser device having a single-peaked FFP, planarization of the optical waveguide becomes important.
Meanwhile, in a GaN-based semiconductor laser, efficiency improvement (power enhancement) is demanded for high-speed write operation and multilayer recording for disk applications. In addition, in display and backlight applications, efficiency improvement (power enhancement) is demanded for brightness enhancement.
If the Eg of an active layer is not constant but varies in an optical gain range of the resonator, the effective volume of the active layer contributing to the generation of laser light decreases. A decrease of the volume of the active layer reduces the gain of the active layer, thereby reducing the efficiency of the laser device. Therefore, in order to achieve a high-efficiency semiconductor laser device, the bandgap energy (Eg) of the active layer needs to be kept sufficiently uniform in the optical gain range.
A method to control an off-angle of the substrate in order to planarize an optical waveguide has been known (see, e.g., Patent Document 1.). For example, a change of an off-angle of the substrate from 0.2° to 1.0° allows planarization of a semiconductor layer formed over the substrate, thereby allowing planarization of the optical waveguide.